Implement support for referenced objects in TSocket.

 -Mods in TMessage
 When a referenced object or a TRef is streamed to a TMessage buffer,
 the bit corresponding to the uuid of the TProcessID is set in the new data 
 member fBitsPIDs.
 -Mods in TSocket.
  Before sending the TMessage containing a referenced object, the list of
 TProcessIDs is sent through the socket before sending the message.
 The list of TProcessIDs already sent is kept in the new member TList* fUUIds.
 When receiving the message, the TProcessID is stored in the global list of
 TProcessIDs if not already there.


git-svn-id: http://root.cern.ch/svn/root/trunk@24168 27541ba8-7e3a-0410-8455-c3a389f83636